Various proposals have been made so far with regard to polymerization catalysts for conjugated dienes and aromatic vinyl compounds, and they play a highly important role in industrial fields. In particular, in order to obtain block copolymers of a conjugated diene and an aromatic vinyl compound with enhanced performance in thermal and mechanical properties, living anionic polymerization utilizing an alkyllithium as a polymerization initiator has been studied and developed.
In the living anionic polymerization, copolymers with different chemical structures having variety of characteristics can be prepared by suitably choosing various polymerization conditions such as addition amounts of the polymerization initiator, amounts and addition time of monomers. The living anionic polymerization is a reaction that is not accompanied by deactivation or novel production of reactive ends by the chain transfer reaction during polymerization, as compared to the radical polymerization, and the reaction is known to have a feature that molecular weight distribution of the resulting polymer is extremely narrower than that of a polymer obtained by the radical polymerization. However, in a polymerization system using an alkyllithium, it is difficult to control stereoregularity of conjugated diene moieties of a copolymer so as to give 1,4-cis-linkages.
Various copolymerization catalysts giving high stereoregularity with 1,4-cis-linkages for conjugated diene segments have been studied and developed so far. For example, complex catalyst systems are known which contain a compound of transition metal such as nickel, cobalt and titanium as a main component (see, Kogyo Kagaku Zasshi (Journal of Industrial Chemistry), 72, 2081, 1969; Plast. Kautsch., 40, 356, 1993; Makromol. Chem. Phys., 195, 2623, 199 etc.), and which contain a compound of rare earth metal such as neodymium and gadolinium as a main component (Macromol. Rapid Commun. 16, 563, 1992; J. Polym. Sci., ParA; Polym. Chem., 32, 1195, 1994; Polymer, 37, 349, 1996) and the like. Although these catalyst systems exhibit a relatively high cis-1,4-controllability, polymers with a high molecular weight and narrow molecular weight distribution, and copolymers with randomized monomer sequence cannot be obtained by means of these catalysts. Further, the polymerization of dienes does not proceed in the manner of living polymerization, and it is extremely difficult to introduce an aromatic vinyl compound to a polymerization terminus.
In order to attain a higher cis-1,4-linkage content and superior polymerization activity, complex catalyst systems which consist of a rare earth metal compound and an organometallic compound belonging to Group I to Group III have been studied and developed, and highly stereospecific polymerization has come to be actively studied (see, Makromol. Chem. Suppl, 4, 61, 1981; J. Polym. Sci., Polym. Chem. Ed., 18, 3345, 1980; German Patent Application No. 2,848,964; Sci. Sinica., 2/3, 734, 1980; Rubber Chem. Technol., 58, 117, 1985 and the like). Among these catalyst systems, complex catalysts containing a neodymium compound and an organoaluminum compound as main components were revealed to give a high cis-1,4-linkage content and have superior polymerization activity. The catalysts have already been used in industrial applications as polymerization catalysts for butadiene and the like (see, Macromolecules, 15, 230, 1982; Makromol. Chem., 94, 119, 1981).
With the recent progress of industrial technologies, requirements for polymeric materials as commercial products have become increasingly higher, and development of polymeric materials which have still higher thermal properties (thermal stability and the like) and mechanical properties (tensile modulus, bending modulus and the like) has come to be strongly desired. As one of promising means for achieving the object, attempts have been made to produce a polymer of a high cis-1,4-configuration content in microstructure and a narrow molecular weight distribution by using a catalyst having a high polymerization activity for conjugated dienes. However, no method has so far been found for producing polymers having such characteristics.